The present invention relates to the field of communications, and, more particularly, to antennas and related methods.
Over the past several years there has been an ever increasing number of frequency bands used for wireless applications. For example, mobile telephones now operate over numerous frequency bands including a variety of cellular frequencies (i.e., in the 800 MHz range) and the personal communications service (PCS) band (i.e., around 1900 MHz).
Since antenna systems are typically configured differently depending upon their intended operating frequency bands, multiple antenna systems would generally be required to monitor and/or communicate over as many frequency bands. This may be an inconvenience for law enforcement and emergency personnel as well as others who need to use multiple frequency bands and would otherwise have to mount multiple antennas on their vehicles to do so.
As a result, some prior art antennas have been designed that may be used with multiple frequency bands. By way of example, U.S. Pat. No. 6,172,651 to Du discloses a window mount vehicle antenna assembly which operates in two frequency bands (e.g., around 800 MHz and 1800 MHz). The antenna assembly includes an inside coupling component mounted on an inside surface of the glass, an outside coupling component mounted on an outside surface of the glass, and a whip antenna element mounted on the outside coupling component. While such antennas may provide increased convenience in that they allow for the use of multiple frequency bands, they may be disadvantageous in certain applications because of the relatively high profile of the relatively long whip antenna element.
Another advantageous feature that may be needed for law enforcement and emergency applications, for example, is the ability to perform direction finding. That is, it may be desirable to locate the direction from which a signal in a particular frequency band is emanating. To do so, an antenna system will require the ability to provide multidirectional beam patterns.
An example of such an antenna is disclosed in U.S. Pat. No. 6,140,972 to Johnston et al. This antenna includes a plurality of radiating elements mounted on a round conducting ground plane. Multiple reflecting surfaces each having a shape of one quarter of a circle or an ellipse are radially disposed about the center of the round ground plane conductor to give a hemispherical shape with multiple equal sectors. Each sector of the antenna includes two types of radiating elements mounted adjacent to the corner of the reflector. The first elemental antenna is responsive to energy having a first polarization, while the second elemental antenna is responsive to energy having a polarization orthogonal to the first polarization. Yet, this antenna has a single operating frequency, and multiple numbers of these antennas would be required to access multiple frequency bands. Further, the use of corner reflectors may increase the overall height profile of the antenna.
Other similar prior art antennas have also been developed which do operate in dual frequency bands. For example, such antennas may include an inner array of monopole antenna elements for operating in the higher of two frequency bands, and an outer array of monopole antenna elements for operating in the lower frequency band. Yet, if the outer array of monopole antenna elements it too tall, it can cause interference (i.e., scattering) with the inner antenna array, which can result in undesirable side lobes in the received signal. Accordingly, the inner antenna arrays of such antennas are generally relatively tall, or even mounted on a raised platform to avoid such interference. As a result, the profile of such antennas may again be too tall for certain applications.
In view of the foregoing background, it is therefore an object of the present invention to provide a multi-frequency band antenna which has a relatively low profile and which allows direction finding.
This and other objects, features, and advantages in accordance with the present invention are provided by a multi-frequency band antenna including a base and first and second antenna arrays. The first antenna array may include a plurality of spaced apart monopole antenna elements extending outwardly from the base a first distance and for operating at a first frequency. Further, the second antenna array may include a plurality of spaced apart antenna elements arranged outside the first antenna array and extending outwardly from the base a second distance less than the first distance. The second antenna array may be for operating at a second frequency lower than the first frequency. Accordingly, the above noted interference problem is significantly reduced, thus reducing the production of undesirable side lobes.
More particularly, each antenna element of the second antenna array may be an annular slotted antenna element. Furthermore, the monopole antenna elements of the first antenna array and the antenna elements of the second antenna array may be omni-directional antenna elements.
Additionally, the base may include an electrically conductive ground plane, and each antenna element of the second antenna array may include a conductive layer and a shaft connecting a medial portion of the conductive layer to the ground plane. Moreover, each antenna element of the second antenna array may further include a feed conductor connected adjacent a peripheral edge of the conductive layer. Each antenna element of the second antenna array may also include a dielectric material (e.g., air or plastic) between an underside of the conductive layer. The conductive layer may have a generally circular shape, for example.
Further, the base may have an upper planar surface so that a lower end of the shaft is in a generally common plane with a lower end of the monopole antenna elements. The multi-frequency band antenna may also include an impedance matching device carried by the base and connected to each antenna element of the second antenna array. Accordingly, blocking or scattering of the higher frequency signals is further reduced.
The plurality of monopole antenna elements of the first antenna array may be arranged at first vertices of a first imaginary regular polygon. Similarly, the plurality of antenna elements of the second antenna array may also be arranged at second vertices of a second imaginary regular polygon concentric with the first imaginary regular polygon. Moreover, the first and second vertices may be equal in number, and the first and second imaginary polygons may be angularly offset from one another.
In addition, the base may include an electrically conductive material to serve as a ground plane for the first and second antenna arrays. A radome may also be included for covering the first and second antenna arrays. Also, a plurality of first controllable phase shifters may be carried by the base for controlling phases of the monopole antenna elements of the first antenna array. Similarly, a plurality of second controllable phase shifters may be carried by the base for controlling phases of the antenna elements of the second antenna array.
A method aspect of the invention is for making a multi-frequency band antenna and may include mounting a plurality of monopole antenna elements on a base in spaced relation and extending outwardly from the base a first distance to form a first antenna array. The first antenna array may be for operating at a first frequency. Furthermore, the method may also include mounting a plurality of antenna elements on the base in spaced relation outside the first antenna array and extending outwardly from the base a second distance less than the first distance to form a second antenna array. The second antenna array may be for operating at a second frequency lower than the first frequency.
Yet another method aspect of the invention is for making a multi-frequency band antenna which may include mounting a plurality of monopole antenna elements in spaced relation on a base and extending outwardly therefrom to form a first antenna array. The method may further include mounting a plurality of annular slotted antenna elements in spaced relation outside the first antenna array on the base and extending outwardly therefrom to form a second antenna array.